Channel state information reporting on licensed and unlicensed carriers

ABSTRACT

A user equipment includes a receiver and a transmitter and is configured to report channel state information (CSI) to a base station in a communication system in which a plurality of downlink component carriers and at least one uplink component carrier are configured. The receiver receives, in a slot nTrigger, a trigger message that triggers reporting of the CSI for at least one unlicensed downlink component carrier of the plurality of downlink component carriers. The transmitter transmits, in a slot nReport, the CSI based on reference signals present in a slot nRS on the at least one unlicensed downlink component carrier. Responsive to the at least one unlicensed downlink component carrier being occupied for a period of time for a bursty downlink transmission, only the reference signals in the slot nRS are evaluated for the CSI, and other slots in the period of time are not evaluated for the CSI.

BACKGROUND 1. Technical Field

The present disclosure relates to methods for reporting channel stateinformation, CSI, from a mobile station to a base station in a mobilecommunication system, particularly on unlicensed carriers. The presentdisclosure is also providing mobile stations for performing the methodsdescribed herein.

2. Description of the Related Art

Long Term Evolution (LTE)

Third-generation mobile systems (3G) based on WCDMA radio-accesstechnology are being deployed on a broad scale all around the world. Afirst step in enhancing or evolving this technology entails introducingHigh-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, alsoreferred to as High Speed Uplink Packet Access (HSUPA), giving a radioaccess technology that is highly competitive.

In order to be prepared for further increasing user demands and to becompetitive against new radio access technologies, 3GPP introduced a newmobile communication system which is called Long Term Evolution (LTE).LTE is designed to meet the carrier needs for high speed data and mediatransport as well as high capacity voice support for the next decade.The ability to provide high bit rates is a key measure for LTE.

The work item (WI) specification on Long-Term Evolution (LTE) calledEvolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial RadioAccess Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTEsystem represents efficient packet-based radio access and radio accessnetworks that provide full IP-based functionalities with low latency andlow cost. In LTE, scalable multiple transmission bandwidths arespecified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order toachieve flexible system deployment using a given spectrum. In thedownlink, Orthogonal Frequency Division Multiplexing (OFDM) based radioaccess was adopted because of its inherent immunity to multipathinterference (MPI) due to a low symbol rate, the use of a cyclic prefix(CP) and its affinity to different transmission bandwidth arrangements.Single-carrier frequency division multiple access (SC-FDMA) based radioaccess was adopted in the uplink, since provisioning of wide areacoverage was prioritized over improvement in the peak data rateconsidering the restricted transmit power of the user equipment (UE).Many key packet radio access techniques are employed includingmultiple-input multiple-output (MIMO) channel transmission techniquesand a highly efficient control signaling structure is achieved in LTERel. 8/9.

LTE Architecture

The overall architecture is shown in FIG. 1 and a more detailedrepresentation of the E-UTRAN architecture is given in FIG. 2. TheE-UTRAN consists of an eNodeB, providing the E-UTRA user plane(PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towardsthe user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY),Medium Access Control (MAC), Radio Link Control (RLC), and Packet DataControl Protocol (PDCP) layers that include the functionality ofuser-plane header-compression and encryption. It also offers RadioResource Control (RRC) functionality corresponding to the control plane.It performs many functions including radio resource management,admission control, scheduling, enforcement of negotiated uplink Qualityof Service (QoS), cell information broadcast, ciphering/deciphering ofuser and control plane data, and compression/decompression ofdownlink/uplink user plane packet headers. The eNodeBs areinterconnected with each other by means of the X2 interface.

The eNodeBs are also connected by means of the S1 interface to the EPC(Evolved Packet Core), more specifically to the MME (Mobility ManagementEntity) by means of the S1-MME and to the Serving Gateway (SGW) by meansof the S1-U. The S1 interface supports a many-to-many relation betweenMMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user datapackets, while also acting as the mobility anchor for the user planeduring inter-eNodeB handovers and as the anchor for mobility between LTEand other 3GPP technologies (terminating S4 interface and relaying thetraffic between 2G/3G systems and PDN GW). For idle state userequipments, the SGW terminates the downlink data path and triggerspaging when downlink data arrives for the user equipment. It manages andstores user equipment contexts, e.g., parameters of the IP bearerservice, network internal routing information. It also performsreplication of the user traffic in case of lawful interception.

The MME is the key control-node for the LTE access-network. It isresponsible for idle mode user equipment tracking and paging procedureincluding retransmissions. It is involved in the beareractivation/deactivation process and is also responsible for choosing theSGW for a user equipment at the initial attach and at time of intra-LTEhandover involving Core Network (CN) node relocation. It is responsiblefor authenticating the user (by interacting with the HSS). TheNon-Access Stratum (NAS) signaling terminates at the MME and it is alsoresponsible for generation and allocation of temporary identities touser equipments. It checks the authorization of the user equipment tocamp on the service provider's Public Land Mobile Network (PLMN) andenforces user equipment roaming restrictions. The MME is the terminationpoint in the network for ciphering/integrity protection for NASsignaling and handles the security key management. Lawful interceptionof signaling is also supported by the MME. The MME also provides thecontrol plane function for mobility between LTE and 2G/3G accessnetworks with the S3 interface terminating at the MME from the SGSN. TheMME also terminates the S6a interface towards the home HSS for roaminguser equipments.

Component Carrier Structure in LTE

The downlink component carrier of a 3GPP LTE system is subdivided in thetime-frequency domain in so-called subframes. In 3GPP LTE each subframeis divided into two downlink slots as shown in FIG. 3, wherein the firstdownlink slot comprises the control channel region (PDCCH region) withinthe first OFDM symbols. Each subframe consists of a given number of OFDMsymbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release8)), wherein each OFDM symbol spans over the entire bandwidth of thecomponent carrier. The OFDM symbols thus each consist of a number ofmodulation symbols transmitted on respective N^(DL) _(RB)*N^(RB) _(sc)subcarriers as also shown in FIG. 4.

Assuming a multi-carrier communication system, e.g., employing OFDM, as,for example, used in 3GPP Long Term Evolution (LTE), the smallest unitof resources that can be assigned by the scheduler is one “resourceblock”. A physical resource block (PRB) is defined as N^(DL) _(symb)consecutive OFDM symbols in the time domain (e.g., 7 OFDM symbols) andN^(RB) _(sc) consecutive subcarriers in the frequency domain asexemplified in FIG. 4 (e.g., 12 subcarriers for a component carrier). In3GPP LTE (Release 8), a physical resource block thus consists of N^(DL)_(symb)*N^(RB) _(sc) resource elements, corresponding to one slot in thetime domain and 180 kHz in the frequency domain (for further details onthe downlink resource grid, see, for example, 3GPP TS 36.211, “EvolvedUniversal Terrestrial Radio Access (E-UTRA); Physical Channels andModulation (Release 8)”, section 6.2, available at http://www.3gpp.organd incorporated herein by reference).

One subframe consists of two slots, so that there are 14 OFDM symbols ina subframe when a so-called “normal” CP (cyclic prefix) is used, and 12OFDM symbols in a subframe when a so-called “extended” CP is used. Forsake of terminology, in the following the time-frequency resourcesequivalent to the same N^(RB) _(sc) consecutive subcarriers spanning afull subframe are called a “resource block pair”, or equivalent “RBpair” or “PRB pair”.

The term “component carrier” refers to a combination of several resourceblocks in the frequency domain. In future releases of LTE, the term“component carrier” is no longer used; instead, the terminology ischanged to “cell”, which refers to a combination of downlink andoptionally uplink resources. The linking between the carrier frequencyof the downlink resources and the carrier frequency of the uplinkresources is indicated in the system information transmitted on thedownlink resources.

Similar assumptions for the component carrier structure apply to laterreleases too.

Carrier Aggregation in LTE-A for Support of Wider Bandwidth

The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequencyspectrum for IMT-Advanced was decided, the actual available frequencybandwidth is different according to each region or country. Followingthe decision on the available frequency spectrum outline, however,standardization of a radio interface started in the 3rd GenerationPartnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the StudyItem description on “Further Advancements for E-UTRA (LTE-Advanced)” wasapproved. The study item covers technology components to be consideredfor the evolution of E-UTRA, e.g., to fulfill the requirements onIMT-Advanced.

The bandwidth that the LTE-Advanced system is able to support is 100MHz, while an LTE system can only support 20 MHz. Nowadays, the lack ofradio spectrum has become a bottleneck of the development of wirelessnetworks, and as a result it is difficult to find a spectrum band whichis wide enough for the LTE-Advanced system. Consequently, it is urgentto find a way to gain a wider radio spectrum band, wherein a possibleanswer is the carrier aggregation functionality.

In carrier aggregation, two or more component carriers are aggregated inorder to support wider transmission bandwidths up to 100 MHz. Severalcells in the LTE system are aggregated into one wider channel in theLTE-Advanced system which is wide enough for 100 MHz even though thesecells in LTE may be in different frequency bands.

All component carriers can be configured to be LTE Rel. 8/9 compatible,at least when the bandwidth of a component carrier does not exceed thesupported bandwidth of a LTE Rel. 8/9 cell. Not all component carriersaggregated by a user equipment may necessarily be Rel. 8/9 compatible.Existing mechanism (e.g., barring) may be used to avoid Rel-8/9 userequipments to camp on a component carrier.

A user equipment may simultaneously receive or transmit one or multiplecomponent carriers (corresponding to multiple serving cells) dependingon its capabilities. A LTE-A Rel. 10 user equipment with receptionand/or transmission capabilities for carrier aggregation cansimultaneously receive and/or transmit on multiple serving cells,whereas an LTE Rel. 8/9 user equipment can receive and transmit on asingle serving cell only, provided that the structure of the componentcarrier follows the Rel. 8/9 specifications.

Carrier aggregation is supported for both contiguous and non-contiguouscomponent carriers with each component carrier limited to a maximum of110 Resource Blocks in the frequency domain using the 3GPP LTE (Release8/9) numerology.

It is possible to configure a 3GPP LTE-A (Release 10) compatible userequipment to aggregate a different number of component carriersoriginating from the same eNodeB (base station) and of possiblydifferent bandwidths in the uplink and the downlink. The number ofdownlink component carriers that can be configured depends on thedownlink aggregation capability of the UE. Conversely, the number ofuplink component carriers that can be configured depends on the uplinkaggregation capability of the UE. It may currently not be possible toconfigure a mobile terminal with more uplink component carriers thandownlink component carriers.

In a typical TDD deployment, the number of component carriers and thebandwidth of each component carrier in uplink and downlink is the same.Component carriers originating from the same eNodeB need not provide thesame coverage.

The spacing between centre frequencies of contiguously aggregatedcomponent carriers shall be a multiple of 300 kHz. This is in order tobe compatible with the 100 kHz frequency raster of 3GPP LTE (Release8/9) and at the same time preserve orthogonality of the subcarriers with15 kHz spacing. Depending on the aggregation scenario, the n*300 kHzspacing can be facilitated by insertion of a low number of unusedsubcarriers between contiguous component carriers.

The nature of the aggregation of multiple carriers is only exposed up tothe MAC layer. For both uplink and downlink there is one HARQ entityrequired in MAC for each aggregated component carrier. There is (in theabsence of SU-MIMO for uplink) at most one transport block per componentcarrier. A transport block and its potential HARQ retransmissions needto be mapped on the same component carrier.

The Layer 2 structure with activated carrier aggregation is shown inFIG. 5 and FIG. 6 for the downlink and uplink respectively.

When carrier aggregation is configured, the mobile terminal only has oneRRC connection with the network. At RRC connectionestablishment/re-establishment, one cell provides the security input(one ECGI, one PCI and one ARFCN) and the non-access stratum mobilityinformation (e.g., TAI) similarly as in LTE Rel. 8/9. After RRCconnection establishment/re-establishment, the component carriercorresponding to that cell is referred to as the downlink Primary Cell(PCell). There is always one and only one downlink PCell (DL PCell) andone uplink PCell (UL PCell) configured per user equipment in connectedstate. Within the configured set of component carriers, other cells arereferred to as Secondary Cells (SCells); with carriers of the SCellbeing the Downlink Secondary Component Carrier (DL SCC) and UplinkSecondary Component Carrier (UL SCC).

The configuration and reconfiguration, as well as addition and removal,of component carriers can be performed by RRC. Activation anddeactivation is done via MAC control elements. At intra-LTE handover,RRC can also add, remove, or reconfigure SCells for usage in the targetcell. When adding a new SCell, dedicated RRC signaling is used forsending the system information of the SCell, the information beingnecessary for transmission/reception (similarly as in Rel-8/9 forhandover).

When a user equipment is configured with carrier aggregation there is atleast one pair of uplink and downlink component carriers that is alwaysactive. The downlink component carrier of that pair might be alsoreferred to as ‘DL anchor carrier’. Same applies also for the uplink.

When carrier aggregation is configured, a user equipment may bescheduled on multiple component carriers simultaneously but at most onerandom access procedure shall be ongoing at any time. Cross-carrierscheduling allows the PDCCH of a component carrier to schedule resourceson another component carrier. For this purpose a component carrieridentification field is introduced in the respective DCI formats, calledCIF.

A linking, established by RRC signaling, between uplink and downlinkcomponent carriers allows identifying the uplink component carrier forwhich the grant applies when there is no-cross-carrier scheduling. Thelinkage of downlink component carriers to uplink component carrier doesnot necessarily need to be one to one. In other words, more than onedownlink component carrier can link to the same uplink componentcarrier. At the same time, a downlink component carrier can only link toone uplink component carrier.

Channel State Information Feedback Elements

Commonly, mobile communication systems define special control signallingthat is used to convey the channel quality feedback. In 3GPP LTE, thereexist three basic elements which may or may not be given as feedback forthe channel quality. These channel quality elements are:

MCSI: Modulation and Coding Scheme Indicator, sometimes referred to asChannel Quality Indicator (CQI) in the LTE specification

PMI: Precoding Matrix Indicator

RI: Rank Indicator

The MCSI suggests a modulation and coding scheme that should be used fortransmission, while the PMI points to a pre-coding matrix/vector that isto be employed for spatial multiplexing and multi-antenna transmission(MIMO) using a transmission matrix rank that is given by the RI. Detailsabout the involved reporting and transmission mechanisms are given inthe following specifications to which it is referred for further reading(all documents available at http://www.3gpp.org and incorporated hereinby reference):

3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA);Physical channels and modulation”(3GPP TS 36.211, “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation”),version 10.0.0, particularly sections 6.3.3 and 6.3.4;3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA);Multiplexing and channel coding”(3GPP TS 36.212, “Evolved UniversalTerrestrial Radio Access (E-UTRA); Multiplexing and channel coding”,version 10.0.0), version 10.0.0, particularly sections 5.2.2, 5.2.4, and5.3.3; and3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA);Physical layer procedures”(3GPP TS 36.213, “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical layer procedures”, version10.0.1), version 10.0.1, particularly sections 7.1.7 and 7.2.

In 3GPP LTE, not all of the above identified three channel qualityelements are necessarily reported at the same time. The elements beingactually reported depend mainly on the configured reporting mode. Itshould be noted that 3GPP LTE also supports the transmission of twocodewords (i.e., two codewords of user data (transport blocks) may bemultiplexed to and transmitted in a single sub-frame), so that feedbackmay be given either for one or two codewords. Some details are providedin the next sections and in Table 1 below for an exemplary scenariousing a 20 MHz system bandwidth. It should be noted that thisinformation is based on 3GPP TS 36.213, section 7.2.1 mentioned above.

The individual reporting modes for the aperiodic channel qualityfeedback are defined in 3GPP LTE as follows:

Reporting Mode 1-2

Contents of this Report:

One set S (i.e., wideband) MCSI value per codeword

One precoding matrix, represented by a PMI, for each subband

In some cases (See 3GPP TS 36.213 clause 7.2.1), additionally one set S(i.e., wideband) PMI value

In case of transmission modes 4, 8, 9, and 10: One RI value

Reporting Mode 2-0

Contents of this Report:

One set S (i.e., wideband) MCSI value

Positions of M selected subbands

One MCSI value for the M selected subbands (2 bits differential to theset S MCSI value, non-negative)

In case of transmission mode 3: One RI value

Reporting Mode 2-2

Contents of this Report:

One set S (i.e., wideband) MCSI value per codeword

One preferred PMI for set S (i.e., wideband)

Positions of M selected subbands

One MCSI value for the M selected subbands per codeword (2 bitsdifferential to the corresponding set S MCSI value, non-negative)

One preferred PMI for M selected subbands—In some cases (See 3GPP TS36.213 clause 7.2.1), additionally one set S (i.e., wideband) PMI value

In case of transmission modes 4, 8, 9, and 10: One RI value

Reporting Mode 3-0

Contents of this Report:

One set S (i.e., wideband) MCSI value

One MCSI value per subband (2 bits differential to set S MCSI value)

In case of transmission mode 3: One RI value

Reporting Mode 3-1

Contents of this Report:

One set S (i.e., wideband) MCSI value per codeword

One preferred PMI for set S (i.e., wideband)

In some cases (See 3GPP TS 36.213 clause 7.2.1), additionally one set S(i.e., wideband) PMI value

One MCSI value per codeword per subband (2 bits differential to themcorresponding set S MCSI value)

In case of transmission modes 4, 8, 9, and 10: One RI value

Reporting Mode 3-2

Contents of this Report:

One set S (i.e., wideband) MCSI value per codeword

One precoding matrix, represented by a PMI, for each subband—In somecases (See 3GPP TS 36.213 clause 7.2.1), additionally one set S (i.e.,wideband) PMI value

One MCSI value per codeword per subband (2 bits differential to thecorresponding set S MCSI value)

In case of transmission modes 4, 8, 9, and 10: One RI value

It should be noted that the term subband is here used so as to representa number of resource blocks as outlined earlier, while the term set Srepresents generally a subset of the whole set of resource blocks in thesystem bandwidth. In the context of 3GPP LTE and LTE-A, the set S so faris defined to always represent the whole cell, i.e., component carrierbandwidth, a frequency range of up to 20 MHz, and is for simplicityhereafter referred to as “wideband”.

Aperiodic & Periodic CQI Reporting

The periodicity and frequency resolution to be used by a UE to reportthe CSI are both controlled by the eNodeB. The Physical Uplink ControlChannel (PUCCH) is used for periodic CSI reporting only; the PUSCH isused for aperiodic reporting of the CSI, whereby the eNodeB specificallyinstructs the UE to send an individual CSI report embedded into aresource which is scheduled for uplink data transmission.

In order to acquire CSI information quickly, eNodeB can scheduleaperiodic CSI by setting a CSI request bit in an uplink resource grantsent on the Physical Downlink Control Channel.

In 3GPP LTE, a simple mechanism is foreseen to trigger the so-calledaperiodic channel quality feedback from the user equipment. An eNodeB inthe radio access network sends an L1/L2 control signal to the userequipment to request the transmission of the so-called aperiodic CSIreport (see 3GPP TS 36.212, section 5.3.3.1.1 and 3GPP TS 36.213,section 7.2.1 for details). Another possibility to trigger the provisionof aperiodic channel quality feedback by the user equipments is linkedto the random access procedure (see 3GPP TS 36.213, section 6.2).

Whenever a trigger for providing channel quality feedback is received bythe user equipment, the user equipment subsequently transmits thechannel quality feedback to the eNodeB. Commonly, the channel qualityfeedback (i.e., the CSI report) is multiplexed with uplink (user) dataon the Physical Uplink Shared CHannel (PUSCH) resources that have beenassigned to the user equipment by the L1/L2 signal (such as the PDCCH)which triggered the channel quality feedback.

Downlink Reference Signals

In the LTE downlink, five different types of RSs are provided:

Cell-specific RSs (often referred to as ‘common’ RSs, as they areavailable to all UEs in a cell and no UE-specific processing is appliedto them);

UE-specific RSs (introduced in Release 8 and extended in release 9 and10), which may be embedded in the data for specific UEs (also known asDemodulation Reference Signals—DM-RSs).

MBSFN-specific RSs, which are used only for Multimedia Broadcast SingleFrequency Network (MBSFN) operation.

Positioning RSs, which from Release 9 onwards may be embedded in certain‘positioning subframes’ for the purpose of UE location measurements.

Channel State Information, CSI, RSs, which are introduced in release 10specifically for the purpose of estimating the downlink channel stateand not for data demodulation.

Each RS pattern is transmitted from an antenna port at the eNodeB. Anantenna port may in practice be implemented either as a single physicaltransmit antenna, or as a combination of multiple physical antennaelements. In either case, the signal transmitted from each antenna portis not designed to be further deconstructed by the UE receiver:

The transmitted RS corresponding to a given antenna port defines theantenna port from the point of view of the UE, and enables the UE toderive a channel estimate for all data transmitted on that antennaport—regardless of whether it represents a single radio channel from onephysical antenna or a composited channel from a plurality of physicalantenna elements together comprising the antenna port.

Cell-Specific Reference Signals

The cell specific RSs enable the UE to determine the phase reference fordemodulating the downlink control channels and the downlink data in mosttransmission modes of the Physical Downlink Share Channel, PDSCH. IfUE-specific pre-coding is applied to the PDSCH data symbols beforetransmission downlink control signaling is provided to inform the UE ofthe corresponding phase adjustment is should apply relative to the phasereference provided by the cell-specific reference signals.

In an OFDM-based system an equidistant arrangement of reference symbolsin the lattice structure achieves the Minimum Mean-Squared Error (MMSE)estimate of the channel. Moreover, in the case of a uniform referencesymbol grid, a ‘diamonds shape’ in the time-frequency plane can be shownto be optimal.

In LTE, the arrangement of REs on which the cell-specific RSs aretransmitted follows these principles. FIG. 7 illustrates the RSarrangement where the cell-specific reference signals are indicated withP, namely, as {P0, P1, P2, P3}.

Up to four cell-specific antenna ports, numbered 0-3, may be used in LTEeNodeB, thus requiring the UE to derive up to four separate channelestimates. For each antenna port, a different RS pattern has beendesigned, with particular attention having been given to theminimization of intra-cell interference between the multiple transmitantenna ports.

In FIG. 7, Px indicates that the RE is used for the transmission of anRS on antenna port X. Then an RE is used to transmit an RS on oneantenna port, the corresponding RE on the other antenna ports is set tozero to limit the interference.

Downlink Reference Signals for Estimation of Channel State Information(CSI-RS)

The main goal of CSI-RS is to obtain channel state feedback for up toeight transmit antenna ports to assist the eNodeB in its precodingoperation. LTE release 10 supports transmission of CSI-RS for 1, 2, 4,and 8 transmit antenna ports. CSI-RS also enables the UE to estimate theCSI for multiple cells rather than just one serving cell, to supportfuture multi-cell cooperative transmission schemes.

The following general design principles can be identified for CSI-RS:

In the frequency domain, uniform spacing of CSI-RS location is highlydesirable:

In the time domain, it is desirable to minimize the number of subframescontaining CSI-RS, so that a UE can estimate the CSI for differentantenna ports and even different cells with minimum wake-up duty calyclewhen the UE is in Discontinuous Reception (DRX) mode, to preservebattery life.The overall CSI-RS overhead involves a trade-off between accuracy CSIestimation for efficient operation and minimizing the impact on legacypre-Release 10 UEs which are unaware of the presence of CSI-RS and whosedata are punctured by the CSI-RS transmission.CSI-RS of different antenna ports within a cell, and, as far aspossible, form different cells, should be orthogonally multiplexed toenable accuracy CSI estimation.

Taking these considerations into account, the CSI-RS patterns selectedfor Release 10 are shown in FIG. 7. CDM codes of length 2 are used sothe CSI-RS on two antenna ports share two REs on a given subcarrier.

The pattern shown in FIG. 7 can be used in both frame structure 1(Frequency Division Duplex, FDD), and frame structure 2 (Time DivisionDuplex, TDD). The REs used for CSI-RSs are labeled RS and used togetherwith the following table grouping the CSI-RS into CSI reference signalconfiguration.

In addition the following table includes for each CSI reference signalconfiguration an identification of the cell index as one of the set of{A, B, C, D, E, F, G, H, I, J, K, L, V, N, O, P, Q, R, S, T} or a subsetthereof and the antenna port and the maximum 8 antenna ports groupedinto CDM groups {x, z, y, u}.

TABLE 1 Number of CSI reference signals configured 1 or 2 4 8 CSI CellCell Cell reference index, index, index, signal CDM COM CDMconfiguration (k′, l′) n_(s) mod 2 group (k′, l′) n_(s) mod 2 group (k′,l′) n_(s) mod 2 group Frame 0 (9, 5) 0 Ax (9, 5) 0 Ax (9, 5) 0 Axstructure 1 (11, 2)  1 Bx (11, 2)  1 Bx (11, 2)  1 Bx type 2 (9, 2) 1 Cx(9, 2) 1 Cx (9, 2) 1 Cx 1 and 2 3 (7, 2) 1 Dx (7, 2) 1 Dx (7, 2) 1 Dx 4(9, 5) 1 Ex (9, 5) 1 Ex (9, 5) 1 Ex 5 (8, 5) 0 Fx (8, 5) 0 Fx (Az) 6(10, 2)  1 Gx (10, 2)  1 Gx (Bz) 7 (8, 2) 1 Hx (8, 2) 1 Hx (Cz) 8 (6, 2)1 Ix (6, 2) 1 Ix (Dz) 9 (8, 5) 1 Jx (8, 5) 1 Jx (Ez) 10 (3, 5) 0 Kx (Ay)(Ay) 11 (2, 5) 0 Lx (Fy) (Au) 12 (5, 2) 1 Vx (By) (By) 13 (4, 2) 1 Hx(Gy) (Bu) 14 (3, 2) 1 Ox (Cy) (Cy) 15 (2, 2) 1 Px (Hy) (Cu) 16 (1, 2) 1Qx (Dy) (Dy) 17 (0, 2) 1 Rx (Iy) (Du) 18 (3, 5) 1 Sx (Ey) (Ey) 19 (2, 5)1 Tx (Jy) (Eu) Frame 20 (11, 1)  1 Ax (11, 1)  1 Ax (11, 1)  1 Axstructure 21 (9, 1) 1 Bx (9, 1) 1 Bx (9, 1) 1 Bx type 22 (7, 1) 1 Cx(7, 1) 1 Cx (7, 1) 1 Cx 2 only 23 (10, 1)  1 Dx (10, 1)  1 Dx (Az) 24(8, 1) 1 Ex (8, 1) 1 Ex (Bz) 25 (6, 1) 1 Fx (6, 1) 1 Fx (Cz) 26 (5, 1) 1Gx (Ay) (Ay) 27 (4, 1) 1 Hx (Dy) (Au) 28 (3, 1) 1 Ix (By) (By) 29 (2, 1)1 Jx (Ey) (Bu) 30 (1, 1) 1 Kx (Cy) (Cy) 31 (0, 1) 1 Lx (Fy) (Cu)

The table corresponds to that included 3GPP TS 36.211 V12.3.0 undersection 6.10.5.2 in Table 6.10.5.2-1: illustrating a mapping from CSIreference signal configuration to (k′, I′) for normal cyclic prefix,additionally including the identification of Cell index, CDM group.

In addition, Cell index and CDM group entries in brackets are meant toindicate which index/group combination corresponds to which RE location(k′, I′) within the time/frequency grids of a resource block; but it isnot intended to indicate that a corresponding CSI reference signalconfiguration index is supported, rather the dependency followsimplicitly from other parts of 3GPP TS 36.211 V12.3.0. Consequently,those entries in brackets should be understood just for illustrationalpurposes.

Layer 1/Layer 2 (L1/L2) Control Signaling

In order to inform the scheduled users about their allocation status,transport format and other transmission-related information (e.g., HARQinformation, transmit power control (TPC) commands), L1/L2 controlsignaling is transmitted on the downlink along with the data. L1/L2control signaling is multiplexed with the downlink data in a subframe,assuming that the user allocation can change from subframe to subframe.

It should be noted that user allocation might also be performed on a TTI(Transmission Time Interval) basis, where the TTI length can be amultiple of the subframes. The TTI length may be fixed in a service areafor all users, may be different for different users, or may even bedynamic for each user. Generally, the L1/2 control signaling need onlybe transmitted once per TTI. Without loss of generality, the followingassumes that a TTI is equivalent to one subframe.

The L1/L2 control signaling is transmitted on the Physical DownlinkControl Channel (PDCCH). A PDCCH carries a message as Downlink ControlInformation (DCI), which in most cases includes resource assignments andother control information for a mobile terminal or groups of UEs. Ingeneral, several PDCCHs can be transmitted in one subframe.

It should be noted that in 3GPP LTE, assignments for uplink datatransmissions, also referred to as uplink scheduling grants or uplinkresource assignments, are also transmitted on the PDCCH. Furthermore,Release 11 introduced an EPDCCH that fulfills basically the samefunction as the PDCCH, i.e., conveys L1/L2 control signaling, eventhough the detailed transmission methods are different from the PDCCH.

Further details can be found particularly in the current versions of3GPP TS 36.211 and 36.213, incorporated herein by reference.Consequently, most items outlined in the background and the embodimentsapply to PDCCH as well as EPDCCH, or other means of conveying L1/L2control signals, unless specifically noted.

Generally, the information sent on the L1/L2 control signaling forassigning uplink or downlink radio resources (particularly LTE(-A)Release 10) can be categorized to the following items:

User identity, indicating the user that is allocated. This is typicallyincluded in the checksum by masking the CRC with the user identity;

Resource allocation information, indicating the resources (ResourceBlocks, RBs) on which a user is allocated. Alternatively thisinformation is termed resource block assignment (RBA). Note that thenumber of RBs on which a user is allocated can be dynamic;Carrier indicator, which is used if a control channel transmitted on afirst carrier assigns resources that concern a second carrier, i.e.,resources on a second carrier or resources related to a second carrier;(cross carrier scheduling);Modulation and coding scheme that determines the employed modulationscheme and coding rate;HARQ information, such as a new data indicator (NDI) and/or a redundancyversion (RV) that is particularly useful in retransmissions of datapackets or parts thereof;Power control commands to adjust the transmit power of the assigneduplink data or control information transmission;Reference signal information such as the applied cyclic shift and/ororthogonal cover code index, which are to be employed for transmissionor reception of reference signals related to the assignment;Uplink or downlink assignment index that is used to identify an order ofassignments, which is particularly useful in TDD systems;Hopping information, e.g., an indication whether and how to applyresource hopping in order to increase the frequency diversity;CSI request, which is used to trigger the transmission of channel stateinformation in an assigned resource; andMulti-cluster information, which is a flag used to indicate and controlwhether the transmission occurs in a single cluster (contiguous set ofRBs) or in multiple clusters (at least two non-contiguous sets ofcontiguous RBs). Multi-cluster allocation has been introduced by 3GPPLTE-(A) Release 10.

It is to be noted that the above listing is non-exhaustive, and not allmentioned information items need to be present in each PDCCHtransmission depending on the DCI format that is used.

Downlink control information occurs in several formats that differ inoverall size and also in the information contained in their fields. Thedifferent DCI formats that are currently defined for LTE are as followsand described in detail in 3GPP TS 36.212, “Multiplexing and channelcoding”, section 5.3.3.1 (current version v12.2.0 available athttp://www.3gpp.org and incorporated herein by reference). In addition,for further information regarding the DCI formats and the particularinformation that is transmitted in the DCI, please refer to thementioned technical standard or to LTE—The UMTS Long Term Evolution—FromTheory to Practice, Edited by Stefanie Sesia, Issam Toufik, MatthewBaker, Chapter 9.3, incorporated herein by reference.

Format 0: DCI Format 0 is used for the transmission of resource grantsfor the PUSCH, using single-antenna port transmissions in uplinktransmission mode 1 or 2.

Format 1: DCI Format 1 is used for the transmission of resourceassignments for single codeword PDSCH transmissions (downlinktransmission modes 1, 2, and 7).

Format 1A: DCI Format 1A is used for compact signaling of resourceassignments for single codeword PDSCH transmissions, and for allocatinga dedicated preamble signature to a mobile terminal for contention-freerandom access (for all transmissions modes).

Format 1B: DCI Format 1B is used for compact signaling of resourceassignments for PDSCH transmissions using closed loop precoding withrank-1 transmission (downlink transmission mode 6). The informationtransmitted is the same as in Format 1A, but with the addition of anindicator of the precoding vector applied for the PDSCH transmission.Format 1C: DCI Format 1C is used for very compact transmission of PDSCHassignments. When format 1C is used, the PDSCH transmission isconstrained to using QPSK modulation. This is used, for example, forsignaling paging messages and broadcast system information messages.Format 1D: DCI Format 1D is used for compact signaling of resourceassignments for PDSCH transmission using multi-user MIMO. Theinformation transmitted is the same as in Format 1B, but instead of oneof the bits of the precoding vector indicators, there is a single bit toindicate whether a power offset is applied to the data symbols. Thisfeature is needed to show whether or not the transmission power isshared between two UEs. Future versions of LTE may extend this to thecase of power sharing between larger numbers of UEs.Format 2: DCI Format 2 is used for the transmission of resourceassignments for PDSCH for closed-loop MIMO operation (transmission mode4).Format 2A: DCI Format 2A is used for the transmission of resourceassignments for PDSCH for open-loop MIMO operation. The informationtransmitted is the same as for Format 2, except that if the eNodeB hastwo transmit antenna ports, there is no precoding information, and forfour antenna ports two bits are used to indicate the transmission rank(transmission mode 3).Format 2B: Introduced in Release 9 and is used for the transmission ofresource assignments for PDSCH for dual-layer beamforming (transmissionmode 8).Format 2C: Introduced in Release 10 and is used for the transmission ofresource assignments for PDSCH for closed-loop single-user or multi-userMIMO operation with up to 8 layers (transmission mode 9).Format 2D: introduced in Release 11 and used for up to 8 layertransmissions; mainly used for COMP (Cooperative Multipoint)(transmission mode 10)Formats 3 and 3A: DCI formats 3 and 3A are used for the transmission ofpower control commands for PUCCH and PUSCH with 2-bit or 1-bit poweradjustments, respectively. These DCI formats contain individual powercontrol commands for a group of UEs.Format 4: DCI format 4 is used for the scheduling of the PUSCH, usingclosed-loop spatial multiplexing transmissions in uplink transmissionmode 2. Transmission Modes for the PDSCH (Physical Downlink SharedChannel)

The Physical Downlink Shared CHannel (PDSCH) is the main data bearingdownlink channel in LTE. It is used for all user data, as well as forbroadcast system information which is not carried on the PBCH, and forpaging messages—there is no specific physical layer paging channel inLTE. Data is transmitted on the PDSCH in units known as Transport Blocks(TBs), each of which corresponds to a Medium Access Control (MAC) layerprotocol data unit (PDU). Transport blocks may be passed down from theMAC layer to the physical layer once per Transmission Time Interval(TTI), where a TTI is one ms, corresponding to the subframe duration.

When employed for user data, one or, at most, two transport blocks canbe transmitted per UE per subframe per component carrier, depending onthe transmission mode selected for the PDSCH for each UE. In LTE,usually there are multiple antennas for downlink, i.e., the eNodeB mayuse multiple transmit antennas, and the UE may use multiple receivingantennas. The two antennas can be used in diverse configurations, whichare distinguished and denoted as transmission modes in LTE. The UE isconfigured by the eNodeB with a particular transmission mode. Forinstance, the single transmission antenna in single receiver antennamode is called transmission mode 1.

The various transmission modes are defined in the 3GPP technicalstandard TS 36.213 (current version 12.3.0), subclause 8.0 for theuplink (particularly Tables 8-3, 8-3A, 8-5, and 8-5A) and subclause 7.1for the downlink (particularly Tables 7.1-1, 7.1-2, 7.1-3, 7.1-5,7.1-5A, 7.1-6, 7.1-6A, and 7.1-7); these are incorporated herein byreference. These tables from 3GPP TS 36.213 show the relationshipbetween RNTI Type (e.g., C-RNTI, SPS C-RNTI, SI-RNTI), the TransmissionMode and the DCI format.

These tables provide several predefined transmission modes identifyingthe particular transmission scheme to be used for the PDSCHcorresponding to the (E)PDCCH.

LTE on Unlicensed Bands—Licensed-Assisted Access LAA

In September 2014, 3GPP initiated a new study item on LTE operation onunlicensed spectrum. The reason for extending LTE to unlicensed bands isthe ever-growing demand for wireless broadband data in conjunction withthe limited amount of licensed bands. Unlicensed spectrum therefore ismore and more considered by cellular operators as a complementary toolto augment their service offering. The advantage of LTE in unlicensedbands compared to relying on other radio access technologies (RAT) suchas Wi-Fi is that complementing the LTE platform with unlicensed spectrumaccess enables operators and vendors to leverage the existing or plannedinvestments in LTE/EPC hardware in the radio and core network.

However, it has to be taken into account that unlicensed spectrum accesscan never match the qualities of licensed spectrum due to the inevitablecoexistence with other radio access technologies (RATs) in theunlicensed spectrum. LTE operation on unlicensed bands will therefore atleast in the beginning be considered rather a complement to LTE onlicensed spectrum than stand-alone operation on unlicensed spectrum.Based on this assumption, 3GPP established the term Licensed AssistedAccess (LAA) for the LTE operation on unlicensed bands in conjunctionwith at least one licensed band. Future stand-alone operation of LTE onunlicensed spectrum without relying on LAA however shall not beexcluded.

The current intended general LAA approach at 3GPP is to make use of thealready specified Rel-12 carrier aggregation (CA) framework as much aspossible where the CA framework configuration comprises a so-calledprimary cell (PCell) carrier and one or more secondary cell (SCell)carriers. CA supports in general both self-scheduling of cells(scheduling information and user data are transmitted on the samecomponent carrier) and cross-carrier scheduling between cells(scheduling information in terms of PDCCH/EPDCCH and user data in termsof PDSCH/PUSCH are transmitted on different component carriers).

A very basic scenario is illustrated in FIG. 8, with a licensed PCell,licensed SCell 1, and various unlicensed SCells 2, 3, and 4 (exemplarilydepicted as small cells). The transmission/reception network nodes ofunlicensed SCells 2, 3, and 4 could be remote radio heads managed by theeNB, or could be nodes that are attached to the network but not managedby the eNB. For simplicity, the connection of these nodes to the eNB orto the network is not explicitly shown in the figure.

At present, the basic approach envisioned at 3GPP is that the PCell willbe operated on a licensed band while one or more SCells will be operatedon unlicensed bands. The benefit of this strategy is that the PCell canbe used for reliable transmission of control messages and user data withhigh quality of service (QoS) demands, such as, for example, voice andvideo, while a PCell on unlicensed spectrum might yield, depending onthe scenario, to some extent significant QoS reduction due to inevitablecoexistence with other RATs.

It has been agreed during RAN1 #78bis, that the LAA investigation at3GPP will focus on unlicensed bands at 5 GHz, although no final decisionis taken. One of the most critical issues is therefore the coexistencewith Wi-Fi (IEEE 802.11) systems operating at these unlicensed bands. Inorder to support fair coexistence between LTE and other technologiessuch as Wi-Fi as well as fairness between different LTE operators in thesame unlicensed band, the channel access of LTE for unlicensed bands hasto abide by certain sets of regulatory rules which depend on region andconsidered frequency band.

A comprehensive description of the regulatory requirements for operationon unlicensed bands at 5 GHz is given in R1-144348, “RegulatoryRequirements for Unlicensed Spectrum” (R1-144348, “RegulatoryRequirements for Unlicensed Spectrum”), Alcatel-Lucent et al., RAN1#78bis, September 2014, incorporated herein by reference. Depending onregion and band, regulatory requirements that have to be taken intoaccount when designing LAA procedures comprise Dynamic FrequencySelection (DFS), Transmit Power Control (TPC), Listen Before Talk (LBT)and discontinuous transmission with limited maximum transmissionduration. The intention of the 3GPP is to target a single globalframework for LAA which basically means that all requirements fordifferent regions and bands at 5 GHz have to be taken into account forthe system design.

DFS is required for certain regions and bands in order to detectinterference from radar systems and to avoid co-channel operation withthese systems. The intention is furthermore to achieve a near-uniformloading of the spectrum. The DFS operation and correspondingrequirements are associated with a master-slave principle. The mastershall detect radar interference, can however rely on another device,that is associated with the master, to implement the radar detection.

The operation on unlicensed bands at 5 GHz is in most regions limited torather low transmit power levels compared to the operation on licensedbands resulting in small coverage areas. Even if the licensed andunlicensed carriers were to be transmitted with identical power, usuallythe unlicensed carrier in the 5 GHz band would be expected to support asmaller coverage area than a licensed cell in the 2 GHz band due toincreased path loss and shadowing effects for the signal. A furtherrequirement for certain regions and bands is the use of TPC in order toreduce the average level of interference caused to other devicesoperating on the same unlicensed band.

Following the European regulation regarding LBT, devices have to performa Clear Channel Assessment (CCA) before occupying the radio channel. Itis only allowed to initiate a transmission on the unlicensed channelafter detecting the channel as free based on energy detection. Theequipment has to observe the channel for a certain minimum during theCCA. The channel is considered occupied if the detected energy levelexceeds a configured CCA threshold. If the channel is classified asfree, the equipment is allowed to transmit immediately. The maximumtransmit duration is thereby restricted in order to facilitate fairresource sharing with other devices operating on the same band.

Considering the different regulatory requirements, it is apparent thatthe LTE specification for operation on unlicensed bands will requireseveral changes compared to the current Rel-12 specification that islimited to licensed band operation.

In connection with the new work item Licensed-Assisted Access it is alsonot finally decided how the mobile station is reporting channel stateinformation, CSI to a base station, particularly in a scenario in whicha plurality of, namely, unlicensed and licensed, component carriers areconfigured for communication between the mobile station and the basestation for at least one of downlink and uplink transmissions. Areliable and efficient CSI reporting mechanism should be implementedtaking into account the special circumstances of unlicensed carriers.

SUMMARY

One non-limiting and exemplary embodiment provides an improved methodfor reporting channel state information, CSI, in a mobile communicationstation, an improved user equipment for reporting the channel stationinformation, CSI, to a base station in the mobile communication system,and a computer readable medium for carrying out the improved method forreporting the CSI in the mobile communication system.

In one general aspect, the techniques disclosed here a method forreporting channel state information, CSI, from a mobile station to abase station in a mobile communication system in which a plurality ofdownlink component carriers and at least one uplink component carrierare configured for communication between the base station and the mobilestation. In this respect, the mobile station receives from the basestation a trigger message that triggers the reporting of channel stateinformation for at least one of the plurality of downlink componentcarriers, the trigger message being received in a subframe n_(Trigger).Then, the mobile station reports to the base station, the triggeredchannel state information for the at least one of the plurality ofdownlink component carriers based on reference signals, RS, present onthe at least one of the plurality of downlink component carriers, in asubframe n_(Report) later than n_(Trigger). The received trigger messageindicates that the reference signals, RS, on the basis of which thechannel state information is to be reported, are present in a subframen_(RS) on the at least one of the plurality of downlink componentcarriers, where n_(Trigger)≤n_(RS)<n_(Report).

According to another aspect, a mobile station is provided for reportingchannel state information (CSI) to a base station in a mobilecommunication system in which a plurality of downlink component carriersand at least one uplink component carrier are configured forcommunication between the base station and the mobile station. Themobile station includes a receiver, which, in operation, receives fromthe base station a trigger message that triggers the reporting of CSIfor at least one of the plurality of downlink component carriers,wherein the trigger message is received in a subframe nTrigger. Themobile station includes a transmitter, which, in operation, transmits tothe base station, the triggered CSI for the at least one of theplurality of downlink component carriers based on reference signalspresent on the at least one of the plurality of downlink componentcarriers, in a subframe nReport later than nTrigger. The referencesignals, on the basis of which the CSI is to be reported, are present ina subframe nRS on the at least one of the plurality of downlinkcomponent carriers, where nRS is indicated by means of higher layersignaling. The triggered CSI may include: a wideband channel qualityindicator (CQI) value per codeword, which is calculated assuming use ofa single precoding matrix in all sub-bands and downlink transmission ona set of sub-bands (S); and a selected single precoding matrix indicator(PMI), or a first and second PMI corresponding to the selected singlePMI. The single PMI may be selected from a codebook subset assumingdownlink transmission on the set of sub-bands (S). The reported PMI andthe reported CQI values may be calculated conditioned on a reported rankindicator (RI) or may be calculated conditioned on RI=1.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary architecture of a 3GPP LTE system;

FIG. 2 shows an exemplary overview of the overall E-UTRAN architectureof 3GPP LTE;

FIG. 3 shows exemplary subframe boundaries on a downlink componentcarrier as defined for 3GPP LTE (Release 8/9);

FIG. 4 shows an exemplary downlink resource grid of a downlink slot asdefined for 3GPP LTE (Release 8/9);

FIG. 5 shows the 3GPP LTE-A (Release 10) Layer 2 structure withactivated carrier aggregation for the downlink;

FIG. 6 shows the 3GPP LTE-A (Release 10) Layer 2 structure withactivated carrier aggregation for the uplink;

FIG. 7 exemplifies a mapping of resource signals onto a PRB indicatingCell-Specific RSs—CRSs indicated as Px, of Demodulation RSs—DM-RSsindicted as Dx, and Channel State Information RSs—CSI-RSs indicated asRS in the downlink of LTE-A, respectively;

FIG. 8 illustrates an exemplary licensed-assisted access scenario, withvarious licensed and unlicensed cells;

FIG. 9 shows a sequence diagram of the improved channel stateinformation reporting mechanism according to one embodiment;

FIG. 10 illustrates a radio communication employing the improved channelstate information reporting mechanism according to an implementation ofthe embodiment;

FIG. 11 shows a first variant of a mapping of CRS and CSI-RS onto a PRBfor use with 8 CSI-RS ports in connection with the improved channelstate information reporting mechanism according to another embodiment;

FIG. 12 shows a second variant of a mapping of CRS and CSI-RS onto a PRBfor use with 2 CSI-RS ports in connection with the improved channelstate information reporting mechanism according to a further embodiment;

FIG. 13 shows a third variant of a mapping of CRS and CSI-RS onto a PRB,wherein FIG. 13 is for 2 CSI-RS ports according to yet anotherembodiment;

FIG. 14 shows a third variant of a mapping of CRS and CSI-RS onto a PRB,wherein FIG. 14 is for 2 CSI-RS ports according to yet anotherembodiment;

FIG. 15 shows a third variant of a mapping of CRS and CSI-RS onto a PRB,wherein FIG. 15 is for 4 CSI-RS ports according to yet anotherembodiment;

FIG. 16 shows a third variant of a mapping of CRS and CSI-RS onto a PRB,wherein FIG. 16 is for 4 CSI-RS ports according to yet anotherembodiment;

FIG. 17 shows a third variant of a mapping of CRS and CSI-RS onto a PRB,wherein FIG. 17 is for 8 CSI-RS ports according to yet anotherembodiment;

FIG. 18 shows a fourth variant of a mapping of CRS and CSI-RS onto a PRBfor use with 8 CSI-RS ports in connection with the improved channelstate information reporting mechanism according to an even furtherembodiment; and

FIG. 19 shows a fifth variant of a mapping of CRS and CSI-RS onto a PRBin set 1 and another PRB in set 2 for use with 8 CSI-RS ports inconnection with the improved channel state information reportingmechanism according to another embodiment.

DETAILED DESCRIPTION

It should be noted that the embodiments may be advantageously used, forexample, in a mobile communication system such as 3GPP LTE-A (Release10/11/12) communication systems as described in the Technical Backgroundsection above, but the embodiments are not limited to its use in thisparticular exemplary communication networks.

A mobile station or mobile node or user terminal or user equipment is aphysical entity within a communication network. One node may haveseveral functional entities. A functional entity refers to a software orhardware module that implements and/or offers a predetermined set offunctions to other functional entities of a node or the network

Nodes may have one or more interfaces that attach the node to acommunication facility or medium over which nodes can communicate.Similarly, a network entity may have a logical interface attaching thefunctional entity to a communication facility or medium over it maycommunicate with other functional entities or correspondent nodes.

The term “radio resources” as used herein has to be broadly understoodas referring to physical radio resources, such as time-frequencyresources.

The term “unlicensed carrier”, and conversely “licensed carrier” are tobe understood in connection with the new LTE work item licensed-assistedaccess (LAA). Correspondingly, “licensed carriers” is the term for thesituation where a carrier is licensed for exclusive use to an operator,usually by a regulatory body that has authority over the radio frequencyusage for a defined geographical region. “Unlicensed carriers” would bethe term used for carrier(s) which cover(s) frequencies which are at themoment not licensed for LTE, and are in particular open for any usagethat complies with certain regulations, or are otherwise sharednon-exclusively. As described in the background section there areseveral differences between licensed carriers and unlicensed carriers,as regards, e.g., reliability, power level and QoS.

The term “higher layer signaling” as used herein has to be understoodbroadly referring to layers above the PHY layer (according to the OSImodel), comprising the MAC layer (e.g., MAC CE), RRC layer, and furtherlayers above, and their corresponding signals and messages.

The term “wideband” in the narrow sense can be understood as spanningthe system bandwidth of, for example, a component carrier. Nevertheless,the term “wideband” as used herein shall not be construed as onlyreferring to configuration where the system bandwidth is covered in itsentirety, namely, of a set of contiguous subbands making up the entiresystem bandwidth; rather, the term “wideband” shall also be understoodas representing a set of adjacent and/or distributed physical resources(such as subbands, resource blocks, or subcarriers).

As explained in the background section, for unlicensed carriers it isnot finally decided how channel state information, CSI, reporting by themobile station is implemented, i.e., how the reporting is carried out ina mobile communication system in which a plurality of downlink(component) carriers and at least one uplink (component) carrier areconfigured for communication between the base station and the mobilestation, where at least one component carrier is available for downlinktransmission. It should be noted that even though in a time domainduplexing scheme a single frequency carrier is used for uplink anddownlink (though not simultaneously), for simplicity of the description,this case should also be understood as having one component carrier foruplink and having one component carrier for downlink.

Specifically, in the background section it has been established thatpresent implementations of channel state information, CSI, reporting aremechanisms that are inadequate considering the regulatory requirementsthat have to be taken into account when designing LAA procedures. Foroperation on unlicensed bands, changes are necessary, particularly tothe current CSI reporting implementation.

When operating an unlicensed carrier as downlink (component) carrier,the presence of continuous, non-interfered reference signals, RS,namely, cell specific reference signals CRS and/or channel stateinformation—reference signals CSI-RS, can no longer be ensured. Theunlicensed carrier access is limited to, for instance, at most 10 mscontinuous usage in Europe.

Further, the unlicensed carrier is supposed to be shared between variousoperators and/or radio access technologies, including, for instance,WIFI. However, coexistence with WIFI nodes on an unlicensed carrier isdifficult as a WIFI node would occupy the unlicensed carrier in itsentirety, assuming full 20 MHz (or even a plurality of 20 MHz carriers)for active transmissions.

Unlicensed carrier access is generally targeting burst transmissions,i.e., a scenario where the unlicensed carrier is occupied for a shortperiod of time for a bursty downlink transmission between a base stationand a mobile station. However, even in this case, the channel stateinformation, and hence the reporting thereof, is crucial for anefficient adaptation to the channel.

The CSI reporting mechanism relies on the presence of reference signalson which the CSI reporting is based. A reference signal is a signalwhich is known to the receiver, and which is inserted into a transmittedsignal at defined positions in order to facilitate channel estimationfor coherent demodulation and measurements.

In the LTE downlink, cell-specific RSs are provided which are availableto all UEs in a cell; UE-specific RSs may be embedded in the data forspecific UEs for purposes of estimating the channel for datademodulation, but not for channel state information reports. In LTERelease 10 support for the transmission of channel state informationreference signals CSI-RS was introduced, with the main goal of obtainingchannel state feedback for up to eight transmit antenna ports to assistthe base station in the precoding operations, and potentially differentresources for measuring the signal strength and the noise+interferencestrength. The configuration of CSI-RS is established by RRC signaling.

Presently, the CSI reporting allows configuration of periodic as well asaperiodic CSI reporting schemes. The CSI reporting is configured by anRRC message for a (i.e., downlink) component carrier. The configurationassumes that the component carrier includes the reference signals on thebasis of which the CSI reporting is effected. In this respect, CSIreporting can in theory be configured for licensed and unlicensedcarriers.

For periodic CSI reporting, the configuration by RRC is sufficient todetermine and initiate the periodic CSI report transmissions. Anaperiodic CSI reporting needs to be triggered on PHY layer, for example,by use of a DCI format 0 message specifying in the “CSI request field”that the transmission of a CSI report is requested. In other words, amessage indicating that a CSI report is requested, may trigger at amobile station the transmission of an aperiodic CSI report.

In this respect, in the direct comparison between the periodic and theaperiodic CSI reporting, the aperiodic CSI reporting appears bettersuited for utilization in connection with unlicensed carriers. Moreover,the transmission of an aperiodic CSI report can be triggered by a basestation which, thereby, can ensure that the regulatory requirements aresatisfied.

Nevertheless, even the aperiodic CSI reporting scheme has disadvantageswhen configured for an unlicensed carrier, namely, for the followingreasons:

The aperiodic CSI reports equally rely on periodic CSI-RS transmissionson the unlicensed carrier. The configuration of CSI-RS presently assumesa same re-occurring mapping to resource elements on a downlink componentcarrier, and the configuration is facilitated exclusively by RRCmessage(s).The aperiodic CSI reports presently combine wideband andfrequency-selective feedback. However, the frequency-selective feedbackis conceptually unsuitable for a shared radio resource, e.g., due toinaccuracies, provided on the unlicensed carrier and only results inunnecessary signaling overhead.The aperiodic CSI report may be based on preceding CRS or CSI-RS whichno longer reflects the channel conditions, in particular for LTE-U burstscenario, where a considerable time-span may elapse between thetransmission of a periodic CSI-RS and the subsequent report instance,leading to inaccuracies due to fluctuations of the channel conditionsdue to, e.g., fading or mobility effects.

The following exemplary embodiments are conceived by the inventors tomitigate the problems explained and to provide a reliable and efficientCSI reporting mechanism, particularly for unlicensed carriers (althoughit is equally applicable to licensed carriers) that are at leastpartially used for downlink communication.

In the following, several exemplary embodiments will be explained indetail. Some of these are supposed to be implemented in thespecification as given by the 3GPP standards and explained partly in thepresent background section, with the particular key features asexplained in the following pertaining to the various embodiments.

It should be noted that the embodiments may be advantageously used, forexample, in a mobile communication system such as 3GPP LTE-A (Release10/11/12) communication systems as described in the Technical Backgroundsection above, but the embodiments are not limited to its use in thisparticular exemplary communication networks.

The explanations should not be understood as limiting the scope of thedisclosure, but as a mere example of embodiments to better understandthe present disclosure. A skilled person should be aware that thegeneral principles of the present disclosure as laid out in the claimscan be applied to different scenarios and in ways that are notexplicitly described herein. Correspondingly, the following scenariosassumed for explanatory purposes of the various embodiments shall notlimit the present disclosure as such.

In the following a set of embodiments will be explained. To simplify theillustration of the underlying principles, several assumptions are made;however, it should be noted that these assumptions should not beinterpreted as limiting the scope of the present application, as broadlydefined by the claims.

According to a first embodiment illustrated in FIG. 9 the improvedchannel state information, CSI, reporting mechanism is provided forreporting CSI by a mobile station to a base station. For the great partof the following description of this first embodiment, it is assumedthat the CSI reporting is performed for an unlicensed carrier. However,the improved CSI reporting mechanism is equally applicable for reportingCSI of a licensed carrier.

It is of fundamental importance to distinguish a “CSI report for acarrier” from a “CSI report on a carrier”; the former denotes thecarrier for which the channel state is reported, while the latterdenotes the carrier on which the channel state information (i.e., thefeedback message) is transmitted. Therefore, the former refers to adownlink carrier (or the time-span when a carrier is usable fordownlink), while the latter refers to an uplink carrier (or thetime-span when a carrier is usable for uplink).

The improved CSI reporting mechanism is preferably carried out in amobile communication system in which a plurality of downlink componentcarriers and at least one uplink component carrier are configuredbetween the base station and the mobile station, even though it can alsobe carried out where only one downlink and one uplink carrier areconfigured. Referring to the terminology of LTE Release 10, thecomponent carriers may equally be referred to as serving cells.

In such a mobile communication system, the mobile station receives (stepS01—FIG. 9) from the base station a trigger message that triggersreporting of channel state information, CSI. By this trigger message, aCSI report is triggered for one or plural of the configured downlinkcomponent carriers. For the following, it shall be assumed that themobile station receives the trigger message in a subframe with indexn_(Trigger).

Notably, the one or plural downlink component carriers, for which theCSI report is triggered, is not necessarily the same downlink componentcarrier on which the trigger message is received. Rather, in oneexemplary scenario the trigger message may be received on a downlinkcomponent carrier corresponding to a licensed carrier and the CSI reportmay be triggered for one or plural downlink component carrierscorresponding to unlicensed carriers.

In an exemplary implementation, the trigger message is in form of adownlink control information, DCI, format, wherein a CSI request fieldindicates that CSI is to be reported by the mobile station. For example,a CSI request field is specified in the DCI format 0 and in the DCIformat 4. In this respect, the triggered channel state information, CSI,is reported aperiodically by the mobile station, namely, as aperiodicCSI report. Aperiodic CSI reporting formats are defined in LTE release10 as PUSCH CSI reporting modes.

In response to the received trigger message, the mobile stationdetermines (step S02—FIG. 9) for the one or plural downlink componentcarriers a CSI report based on reference signals present on thisdownlink component carrier(s). In other words, the trigger messagereferences (or indicates) the one or plural downlink component carrierson the basis of which the CSI report is to be established.

The CSI report is based on the indicated reference signals, RS, presenton the at least one of the downlink component carriers. In other words,the mobile station evaluates the indicated reference signals with whichit is configured and based thereon reports the triggered CSI to the basestation. The RSs used for this purpose are preferably CRS or CSI-RS.

In another exemplary implementation, for the CSI reporting the mobilestation evaluates reference signals of contiguous or distributedphysical resource blocks, PRBs, in the set of sub-bands, S, on the atleast one of the plurality of downlink component carriers for which theCSI is to be reported. The set of sub-bands S is a system parameter withwhich the mobile station is pre-configured.

Even though LTE specifications use the term “sub-band” as a plurality ofphysical resource blocks, the usage here should be construed as notbeing restricted to such a definition. Rather, a sub-band as describedherein can also be an individual physical resource block, or even a partof a physical resource block such as one or a plurality of subcarriers.

In this respect, as the set S may only refer some of the resource blocksof the cell, it is necessary to pay attention to interpreting the termwideband (or set S) used in connection with the embodiments broader thanonly “wideband” (or “set S”) as such. For example, “wideband” shall notbe construed to mean exclusively the whole system bandwidth, but ratherthe plurality of resource blocks contained in set S, which mayfurthermore be non-adjacent in the frequency domain.

Commonly, the set of sub-bands S is configured such that it spans the(e.g., entire) downlink system bandwidth of the one or plural componentcarriers. In this respect, in this exemplary implementation, the CSI isreported as a wideband CSI report for the set of sub-bands S spanningthe (e.g., entire) downlink system bandwidth of the one.

In any case, the configured set of sub-bands S, for which the widebandCSI is reported in this exemplary implementation, differs from afrequency selective CSI report which may be configured in addition oralternative to the wideband CSI report. Moreover, a selective reportingof CSI for specific subbands contradicts the approach of reportingwideband CSI for the (e.g., entire) downlink system bandwidth of the oneor plural component carriers.

Further, in response to the receipt of the trigger message the mobilestation reports (step S03—FIG. 9) the triggered channel stateinformation, CSI, report for the one or plural downlink componentcarriers. For the following, it shall be assumed that the mobile stationtransmits the CSI report in a subframe with index n_(Report) later thann_(Trigger).

Further to the exemplary implementation, the trigger message in the DCIformat carries an uplink resource assignment. Accordingly, the uplinkresource in which the CSI report shall be transmitted is determined bythe L1/L2 control signal carried in the trigger message. In a morespecific example, the trigger message is in DCI format 0 or DCI format4.

Due to the determined uplink resource in the trigger message, the CSIreported on the uplink resource may not be based on reference signalswhich are more recent than the CSI report. In other words, the referencesignals must have been received prior to the subframe n_(Report) inwhich the CSI report is to be transmitted. Accordingly, the subframen_(Report) may be understood as an upper/latest limit to the subframewhere the RSs serving as the reference for the triggered CSI report aretransmitted.

In another exemplary implementation, which can be combined with theabove, the CSI report is reported on the physical uplink shared channel,PUSCH, on the at least one uplink component carrier. In particular, apre-configured CSI reporting mode is used for conveying the CSI reportto the base station.

As already described before, the reference signals on which the CSIreport is based are transmitted in the one or plural downlink componentcarriers for which the mobile station reports the CSI. Specifically, theone or plural downlink component carriers on which the reference signalsare present may be different from the downlink component carrier onwhich the trigger message is transmitted/received.

Advantageously, the trigger message indicates a subframe with indexn_(RS) on which the reference signals, RS, are present. The referencesignals are present on the one or plural downlink component carriersbased on which the CSI is to be reported.

In this respect, the trigger message does not leave open on the basis onwhich reference signals the CSI is to be reported. The indicatedsubframe with index n_(RS) is sometimes also termed “reference resource”referencing the resources for the CSI report.

In a more detailed implementation, the mobile station is adapted tointerpret the received trigger message for triggering a CSI report suchthat it indicates a singular occasion for reference signals, RS, to bepresent in the subframe n_(RS) on the one or plural downlink componentcarriers for which the CSI report is triggered (i.e., requested). Inother words, the mobile station is adapted to utilize the receivedtrigger message for one-shot CSI reporting only.

Exemplarily, the mobile station is adapted to interpret the receivedtrigger message for triggering a CSI report such that it indicatessingular occasions of reference signals, RS, are offset by a predefinedor signalled subframe number 11 with respect to the trigger message inn_(Trigger). In this respect, the received trigger message indicatesthat the singular occasions of reference signals are present in subframewith index: n_(RS)=n_(Trigger)+I₁, on the one or plural downlinkcomponent carriers for which the CSI report is triggered.

Alternatively, the mobile station is exemplarily adapted to interpretthe received trigger message for triggering a CSI report such that itindicates plural occasions of reference signals, RS; which are offset bya predefined or signalled subframe number 11 with respect to the triggermessage in n_(Trigger) and are spaced at a further predefined orsignalled subframe number I₂.

In this respect, the received trigger message indicates that the pluraloccasions of reference signals are present in subframes with index:n_(RS)=n_(Trigger)+I₁, n_(Trigger)+I₁+I₂, n_(Trigger)+I₁+2*I₂, . . . onthe one or plural downlink component carriers for which the CSI reportis triggered. In other words, the mobile station is adapted to utilizethe received trigger message for plural CSI reports each for arespective one of the plural reference signals in the indicatedsubframes n_(RS).

Specifically, the indication of subframe with index n_(RS) isadvantageous for the CSI reporting of unlicensed carriers. In view ofthe indication of one or plural specific subframe(s) n_(RS) that the CSIreport is to be based on, the base station can reduce the number ofsubframes which have to be evaluated by the mobile station for the CSIreporting, and can at the same time ensure that only the referencesignals of a relevant subframe, namely, of the subframe with indexn_(RS), are evaluated for the CSI reporting.

For best adaptation to the shared nature of an unlicensed carrier, it isadvantageous to indicate n_(RS) in the DCI message triggering thereport. However, since this may require an undesirable overhead, anothersolution is to indicate it by means of higher layer signalling such asRRC messages, preferably within the same message that configures otherCSI report or CSI-RS parameters.

Nevertheless, it shall be emphasized that the trigger message (or asjust mentioned, a higher layer configuration) not necessarily include afield for directly indicating the subframe on which the referencesignals, RS, are present. Instead the trigger message may alsoindirectly reference a subframe based on a prescribed offset betweenn_(RS) and n_(Trigger) or between n_(RS) and n_(Report), i.e., relativeto the subframe with index n_(RS) or n_(Trigger).

According to an exemplary implementation, a predefined CSI reportingmode may foresee that the CSI report for one or plural downlinkcomponent carriers is based on reference signals which are present inthe same subframe as the trigger message but not necessarily on the samedownlink component carrier of the trigger message.

In any case, the received trigger message indicates that the referencesignals, RS, for the CSI report, are in subframe with index n_(RS),where n_(Trigger)≤n_(RS)<n_(Report). Thereby, not only the amount ofbuffering for reference signals reduces but also it can be ensured thatCSI is reported only on the basis of reference signals, RS, from mostrecent subframes.

This keeps the influence of fading effects on the reposted CSI small, sothat inaccuracies between the channel state at the time of measurementand at the time of reporting can be largely avoided; even more so incase only a single CQI value is reported for the set S instead of a CQIvalue per element (e.g., subband) within the set S. In other words, asresult of the restrictions on the indicated subframe index n_(RS), it isnot (i.e., no longer) possible that a CSI report is based on referencesignals present in a subframe with an index n_(RS) before n_(Trigger),and, hence, is outdated and only inaccurately reflects the channelstate.

Implementation in LTE

Now, a more detailed implementation of the above embodiment shall bediscussed in connection with FIG. 10. As shown therein, the improved CSIreporting mechanism is illustrated for a mobile communication system inwhich two downlink component carriers termed “DL Cell 1” and “DL Cell 2”and one uplink component carrier termed “UL Cell 1” are configured forcommunication between the mobile station and the base station.

This detailed implementation utilizes at least for “DL cell 2” the 2CSI-RS port configuration for the transmission of CSI reference signals.Specifically, the mobile station is configured at least for “DL cell 2”with a CSI-RS mapping to resource elements according to CSI RSconfiguration 0 as introduced in the background section, namely,indicating (k′, I′) as (11, 4) as CSI reference signals on the basis ofwhich the CSI reporting is to be carried out.

Further, the detailed implementation assumes the utilization of CSIreporting mode which prescribes an n_(RS) to n_(Trigger) offset equal tozero. In other words, for purposes of CSI reporting the mobile stationis configured to refer to CSI reference signals in the same subframe asthe subframe in which the CSI request (i.e., the trigger message) wasreceived.

In the detailed implementation, the mobile station receives on the PDCCHof “DL Cell 1” in subframe n_(Trigger) a DCI format 0 as trigger messageincluding the “CSI request field” indicating (by a value of ‘1’, ‘01’,‘10’, or ‘11’, depending on the length of the CSI request field andcorresponding higher layer configuration; see 3GPP TS 36.213 v12.3.0clause 7.2.1) that an aperiodic CSI report is triggered. At the sametime the trigger message indicates that the CSI is to be reported for“DL Cell 2”. Further, the trigger message in DCI format 0 indicates anuplink resource assignment for subframe n_(Report) (which is generallyn_(Trigger)+k, where k>=4) in “UL Cell 1”.

Applying the configuration to refer to the presence of CSI referencesignals in the same subframe, i.e., assuming n_(RS)=n_(Trigger), themobile station refers to CSI reference signals according to CSI RSconfiguration 0 of “DL Cell 2” within the same subframe as the triggermessage, namely, within same subframe n_(Trigger), formeasuring/determining the CSI report. On the basis of this CSI referencesignals in subframe n_(RS) on “DL Cell 2”, the mobile station determinesthe triggered aperiodic CSI report.

Subsequently, the mobile station transmits the CSI report in thesubframe n_(Report) (=n_(Trigger)+k) on the uplink resources indicatedin the uplink resource assignment of the DCI format 0 trigger message.Moreover, the mobile station is reporting the aperiodic CSI on the basisof CSI reference signals that are present in the subframe n indicatedthrough the received DCI format 0 trigger message.

Aperiodic CSI Reporting Mode

Now, reference is made to a specific implementation of the aperiodic CSIreport which shall be understood as a new aperiodic CSI reporting modewhich differs from the modes for CSI reporting using PUSCH disclosed in3GPP TS 36.213 V12.3.0, section 7.2.1. This CSI reporting mode ispreferable for an unlicensed carrier. However, it shall be understoodthat this aperiodic CSI reporting mode is equally applicable forlicensed carriers and, hence, shall not be limited in this respect.

This aperiodic CSI reporting mode assumes the reporting of wideband CSI.In other words, reference signals, RS, on the basis of which theaperiodic CSI reporting is performed, are present in a set of sub-bands,S, which is a subset of or spans the downlink system bandwidth of theone or plural downlink component carriers for which the aperiodic CSIreport is triggered. It should be noted that the set S can be specificto each downlink component carrier.

More specifically, the aperiodic CSI reporting is carried out in form ofone or two channel quality indicator, CQI, value(s) for the set ofsub-bands, S, which is a subset of or spans the downlink systembandwidth of the at least one of the plurality of downlink componentcarriers. Whether one or two CQI value(s) are reported for the set ofsub-bands, S, depends on the rank indicator, RI, which is configured forthe mobile station.

It is also possible that a first set of subbands S1 of resources whereRS are present is different from the second set S2 of resources forwhich a CQI value is reported. For example, it may be preferable totransmit RS only on a first subset of the downlink system bandwidth of adownlink component carrier, while the CQI value is determined assumingthat the measured channel state is applicable to the whole downlinkbandwidth of a downlink component carrier and that transmission wouldoccur on the whole downlink bandwidth of the downlink component carrier.

This is similar to obtaining a limited number of samples that isrepresentative of the ensemble or its average. In general, the first andsecond sets of subbands are configurable independently, where preferablythe second set S2 of resources where transmission is assumed is asuperset of, or equal to, the first set S1 of resources where RS aretransmitted/received.

If the mobile station is not configured to report rank indicator, RI,feedback, or if the to be reported rank indicator equals 1 (RI=1), asingle channel quality indicator, CQI, value (corresponding to acodeword) is reported. Further, if the to be reported rank indicator islarger than one (RI>1) two channel quality indicator values(corresponding to different codewords) are reported.

In this respect, the channel state information CSI reporting mode isreported in the form of:

a wideband channel quality indicator, CQI, value per codeword which iscalculated assuming downlink transmission using a single precodingmatrix in a first set of sub-bands and that the reference signals arepresent on a second set of sub-bands; and

a selected precoding matrix indicator, PMI, or a first and secondprecoding matrix indicator corresponding to the selected singleprecoding matrix.

The single precoding matrix is selected from the codebook subsetassuming downlink transmission on the set of sub-bands, S; and the to bereported precoding matrix indicator, PMI, and the to be reported channelquality indicator, CQI, values are calculated conditioned on thereported rank indicator, RI, or are calculated conditioned on RI=1.

In other words, for aperiodic CSI Reports for an unlicensed carrier, anew aperiodic reporting mode is defined, as follows:

A single precoding matrix is selected from the codebook subset assumingtransmission on a set S of subbands. A UE shall report a wideband CQIvalue per codeword which is calculated assuming the use of the singleprecoding matrix in all subbands and transmission on the set S ofsubbands. The UE shall report the selected single precoding matrixindicator except with 8 CSI-RS ports configured for transmission modes 9and 10 or with alternativeCodeBookEnabledFor4TX-r12=TRUE configured fortransmission modes 8, 9, and 10, in which case a first and secondprecoding matrix indicator are reported corresponding to the selectedsingle precoding matrix. For transmission modes 4, 8, 9, and 10, thereported PMI and CQI values are calculated conditioned on the reportedRI. For other transmission modes they are reported conditioned on rank1.

Mapping of CRS and CSI-RS onto Subframe n_(RS)

Now, reference is made to a specific implementation of the subframe onwhich the reference signals, RS, are present. Reference signals shall beunderstood as at least one of the cell-specific reference signals, CRS,and channel state information reference signals, CSI-RS defined in 3GPPTS 36.211 V12.3.0, section 6.10.1 for CSR and 6.10.5 for CSI-RS.

For the cell-specific reference signals, CRS, the base stationconfigures a cell with a number of so-called CRS ports, which—amongstother purposes—determines the number and location of resource elementswhere CRS are transmitted in a subframe. The resource element locationis further a function of the physical cell ID.

Further, for the channel state information reference signals, CSI-RS, amobile station is configured with one or plural sets of CSI referencesignals. The mapping of CSI reference signal transmissions ispre-configured on a per-subframe basis. Specifically, CSI-RS are usedfor downlink transmission mode 10.

In more detail, a mobile station may presently be configured withmultiple sets of CSI reference signals, namely, up to threeconfigurations for which the mobile station shall assume non-zerotransmission power for the CSI-RS (commonly also referred to asNZP-CSI-RS), and zero or more configurations for which the mobilestation shall assume zero transmission power (commonly referred to asZP-CSI-RS) as defined in TS 36.211 under section 6.10.5.2.

In one exemplary implementation, the CRS or CSI-RS transmissions arepunctured into potential PDSCH resource elements within the samedownlink subframe. This implementation contradicts the general approachthat only those resource elements, REs, can be utilized for PDSCHtransmission which is not reserved for other purposes (i.e., RSs,synchronization signals, PBCH, and control signaling). Hence, puncturingof the PDSCH would contradict this general approach in that the mobilestation would assume at the REs could be reserved for PDSCH but insteadcarry the CRS or CSI-RS.

In other words, in case a physical downlink shared channel, PDSCH,utilizes the same subframe n_(RS) in which the CRS or CSI-RS arepresent, the mobile station assumes a punctured PDSCH transmission.However, this implementation is advantageous in that the PDSCH can bedecoded irrespective of whether the trigger message for triggering a CSIreport on the basis of the CRS or CSI-RS presence was received or not.

In more detail, when a mobile station receives the trigger message for aCSI report then reference signals are indicated for a subframe on thebasis of which the CSI is to be reported. Accordingly, the mobilestation assumes whether or not corresponding REs are carrying CRS orCSI-RS in the indicated subframe, depending on whether or not the mobilestation has received the CSI report indicating the CRS or CSI-RS arepresent in the subframe.

In this respect, should the mobile station have misconceived or missedthe receipt of the CSI trigger indicating that the CRS or CSI-RS arepresent in a specific subframe, the mobile station can validly assumethat the REs are not reserved for other purposes and hence include thepunctured PDSCH.

Even if the REs are conversely carrying the CRS or CSI-RS instead of thePDSCH, the mobile station correctly receives, due to the puncturing, theremainder of the allocated PDSCH. The puncturing prevents from downlinkbuffer corruptions, so that even if CRS or CSI-RS symbols areerroneously included in the decoding of PDSCH, the remaining redundancyprovided by forward error correction of the PDSCH can be sufficient tocompensate for such an error and therefore still result in a successfuldecoding of the PDSCH codeword(s).

Further to the CRS and CSI-RS mapping, the mobile station may beconfigured to use the same or different CRS or CSI-RS to measure thesignal strength, S, and/or the interference plus noise strength, I+N.More particularly, the NZP-CSI-RS are well suited for measurement of thesignal component within the SINR, and the ZP-CSI-RS are well suited formeasurement of the interference plus noise component within the SINR.

Nevertheless, presently the ZP-CSI-RS are configured for channel stateinformation—interference measurement CSI-IM via Radio Resource Control,RRC, layer signalling (also, presently the NZP-CSI-RS are configured byRRC). Instead, in one implementation the improved CSI reportingmechanism indicates reference signals on which the CSI is to be reportedutilizing DCI signalling via the PHY layer, by indicating at least oneof NZP and ZP RS.

In this respect, in a further exemplary implementation, it is proposedthat not only the location (i.e., subframe) of the non-zero-power (NZP)reference signals is indicated in CSI trigger message in form of DCIsignalling, but also the location (i.e., subframe) of the zero-power(ZP) reference signals, namely, CRS and/or CSI-RS, is indicated by thesame CSI triggering message in form of DCI signalling. As describedabove, both indications of a subframe may be direct or indirect, forexample, based on a prescribed offset relative to CSI trigger message.

The NZP reference signals and the ZP reference do not necessarily haveto be in a same subframe. Accordingly, in another exemplaryimplementation, the mobile station is configured with at least onereference signal configuration including at least one of: anon-zero-power CSI-RS configuration for which the mobile station assumesnon-zero transmission power in a subframe n_(NZP-CSI-RS)=n_(RS); and azero-power CSI-RS configuration for which the mobile station assumeszero transmission power in a subframe n_(ZP-CSI-RS)≠n_(RS), and,preferably, the subframe n_(ZP-CSI-RS) is earlier than the subframen_(NZP-CSI-RS), wheren _(Trigger) ≤n _(ZP-CSI-RS) <n _(NZP-CSI-RS) <n _(Report).

Consequently, each of the CSI-RS follow independently the equationdefined with respect to the above described embodiments. Thenon-zero-power and the zero-power reference signals are not in the samesubframe of the one or plural component carriers for which the CSIreporting is triggered but can be carried in different subframes of theone or plural component carriers.

In this respect, burst downlink transmission can reserve more REs in asubframe where only the non-zero-power reference signals are mappedwhereas the non-zero-power reference signals are mapped to a preceding,hence, different silent subframe. In this respect, silent period asspecified in the regulatory requirements can be optimized, and moreresources can be made available for data transmission during an activeperiod.

In a further exemplary implementation, downlink component carrierutilization for unlicensed carriers implementing Listen-Before-Talk,LBT, shall be optimized. In case a CSI report is triggered for such aLBT downlink component carrier, the non-zero-power and the zero-powerreference signals are indicated in the trigger message as discussedbefore. However, a distinction is made with respect to how the basestation gains access to such a LBT downlink component carrier.

The base station has to transmit on the LBT downlink component carrieras soon as its availability is detected (e.g., the base station listenedand detected that it is free) a signal to reserve its usage for “useful”signal transmissions. Nevertheless, in case of plural downlink, namely,licensed and unlicensed, carriers transmissions must generally bealigned between the carriers. Hence, the necessity for carrier alignmentcould prevent the base station from immediately starting “useful” signaltransmissions on the LBT downlink component carrier.

In this respect, the base station will transmit a “reservation signal”for reserving the LBT downlink component carrier prior to a competingnoted blocking the LBT downlink component carrier through itstransmission. In this exemplary embodiment it is proposed that the“reservation signal” includes zero-power resource elements which can beused for interference plus noise measurements by the mobile station, andmay furthermore also include non-zero-power resource elements which canbe used for signal strength measurements by the mobile station.

In other words, as the misalignment prevents “useful” transmissions aspart of the “reservation signal” on the LBT downlink component carrier,the base station is forced to postpone all signal transmissions to thenext radio frame boundary. Nevertheless, the base station can triggerCSI reporting via a different component carrier and indicate thesubframe of the “reservation signal” comprising the zero-power andnon-zero-power resource elements for channel state measurements.

According to a further exemplary implementation, the density of the CRSand/or CSI-RS is reduced in the frequency domain. For example, themapping of CRS and/or CSI-RS is adapted that every second configured CRSand/or CSI-RS (e.g., the even/odd numbered reference signals) aretransmitted in form of non-zero-power CRS and/or non-zero-power CSI-RS;whereas the other configured CRS and/or CSI-RS (e.g., the odd/evennumbered reference signals) are transmitted in form of zero-power CRSand/or zero-power CSI-RS. This mapping can be configured as part of aCSI reporting mode, or beneficially may also be included in the CIStrigger message in form of the DCI.

As another example, some sub-bands might contain zero-power CRS orzero-power CSI-RS, while other sub-bands might contain non-zero-powerCRS or non-zero-power CSI-RS, while yet other sub-bands might contain noCRS or CSI-RS. This serves to keep the resource and power overhead dueto RS transmissions low, increasing the efficiency for datatransmission, and/or increasing the available transmit power for otherresources in the subframe.

According to a further exemplary implementation, the CSI referencesignal configuration includes a zero-power CSI-RS configuration forwhich the mobile station assumes zero transmission power indicating atleast one resource element, RE, in the subframe n_(RS) corresponding toa resource element prescribed for cell specific reference signal, CRS,transmission.

This exemplary implementation is discussed in more detail in connectionwith the following examples of advantageous mapping of CRS and CSI-RS,as illustrated in FIGS. 11 to 19.

For this purpose, no further distinction is made between zero-power andnon-zero-power reference signals since the configuration and mapping isequally applicable to both; likewise no further distinction is madebetween the CRS and CSI-RS for the purpose of estimating the channelstate according to the embodiments and implementations. Consequently,only the term “reference signal” (RS) is utilized in the followingdiscussion and corresponding figures.

According to FIG. 7, the state of the art supports time/frequencyresources for RS predominantly resources I′=2 in a second slot of asubframe, as well as in resource elements (k′, I′)={(2, 5), (2, 6), (3,5), (3, 6), (8, 5), (8, 6), (9, 5), (9, 6)} of each slot of a subframe.P0/P1/P2/P3 denote resource elements for the CRS transmissioncorresponding to ports 0/1/2/3, respectively, while D7-8 and D9-10denote resource elements for the UE-specific RS (or DM-RS) transmissioncorresponding to ports 7-8 and 9-10, respectively. Where applicable,DM-RS for ports 11-12 are mapped to the same resources as for ports 7-8,and DM-RS for ports 13-14 are mapped to the same resources as for ports9-10.

According to an implementation as exemplified in FIG. 11, the RS formeasuring the CSI are mapped onto one or more resource elements that arecandidates for CRS port 0/1/2/3 corresponding to I′=0, 1 in the firstslot of a subframe. This is particularly reasonable if there is no datatransmission in the same subframe, as it enables the earliest possibletransmission time of the RS, and therefore a high amount of time forprocessing is available between the reception of the signal and thecorresponding required CSI report, thereby enabling a relatively simpleimplementation of the measurement and reporting procedures.

It is also reasonable if a PDSCH transmission in the same subframe isdone by a transmission scheme that does not rely on CRS for datademodulation, such as in the “Up to 8 layer transmission scheme”supported by transmission modes 9 and 10. It is furthermore beneficialfor cases that the RS serve not only for obtaining the CSI, but also tooccupy a shared carrier (such as an unlicensed carrier) to block othernodes from accessing the channel; in this case, the RS would serve as akind of “reservation signal”, and is particularly relevant in thebeginning of a bursty access to the unlicensed carrier.

Similarly to the notation found in Sesia S, Toufik I, Baker M, “LTE TheUMTS Long Term Evolution—From Theory to Practice”, Second Edition, 2011,John Wiley & Sons, Ltd., chapter 29.4, in FIG. 29.4, the uppercasecharacter A/B/C denote a given RS configuration, while the lowercasecharacters x/y/z/u denote corresponding antenna ports.

Consequently, FIG. 11 shows possible configurations for 8 RS ports. Incase of configurations for 4 RS ports, corresponding labelsAz/Au/Bz/Bu/Cz/Cu would be replaced by Dx/Dy/Ex/Ey/Fx/Fy, respectively,to allow more different RS configurations. Likewise, for configurationsfor 2 RS ports, additionally corresponding labels Ay/By/Cy/Dy/Ey/Fywould be replaced by Gx/Hx/Ix/Jx/Kx/Lx.

In another implementation, alternative or additional RS configurationscan be supported by mapping RS to the second slot of a subframe. Acorresponding example for additional RS configurations for the case of 2RS ports is shown in FIG. 12, where a total of 24 different RSconfigurations are supported.

Further variants for different mappings are exemplarily shown for thecases of 2, 4, and 8 RS ports in FIGS. 13-17. The figures show the casethat a single resource element carries a single antenna port RS, i.e.,no further CDM multiplexing is necessary to support different ports onthe same resource elements. For this purpose, each RS is represented byan uppercase letter (A/B/C/E) followed by a single number, whereidentical letters correspond to the same RS configuration and the numberindicates the corresponding RS port.

Alternatives in the detailed arrangements are shown, for example, to mapequal ports of different configurations to time-adjacent resourceelements, or to map different ports of the same RS configuration totime-adjacent resource elements.

The former has the technical advantage that a better averaging effectcan be obtained when combining different RS ports to obtain a channelstate estimate, while the latter can expend more transmit power to eachRS port in case, e.g., only one configuration is active per subframe andRB; as can be seen, e.g., for the 2 RS port case and configuration A, insymbol I′=0 of the first slot the full transmit power can be spent onsymbol A1 in case of FIG. 14 while according to FIG. 13 the transmitpower needs to be split among A1 and A2.

FIG. 18 shows another alternative implementation, where not only RS forCSI measurements are transmitted in the beginning and the end of asubframe, but furthermore DM-RS are transmitted in the beginning of thefirst slot of a subframe. Transmitting DM-RS in the beginning of asubframe can give the benefit that these signals not only serve toprovide means to estimate the channel for data demodulation, but also asreservation signals to keep other nodes from assessing the channel asvacant at the beginning of a subframe. Therefore this DM-RS transmissionmethod may be advantageous even in subframe where no RS for CSIestimation (such as CSI-RS or CRS) are transmitted.

In one implementation, the configurations shown exemplarily in FIGS. 11to 18 are understood to be applicable for every resource block withinthe system bandwidth of a downlink component carrier. In anotherimplementation, the configurations are applicable only to a subset ofresource blocks within the system bandwidth of a downlink componentcarrier.

This is particularly applicable to reduce the overall overhead incurredby RS transmissions in a subframe. At the same time, when extending thisprinciple to allowing a first RS configuration in a first set ofresource blocks and a second RS configuration in a second set ofresource blocks, it can be used to effectively support more simultaneousconfigurations in a subframe.

For example, FIG. 19 exemplarily shows two resource blocks of the samesubframe supporting 8 RS ports based on FIG. 18. However, in this way afirst UE can be configured with RS configuration A in resource block set1, while a second UE can be configured with RS configuration B inresource block set 2.

Even though configurations A and B can be identical with respect to theresource element location within the resource block for the RStransmission, they can be used for different UEs due to the differentdetailed configuration for the UEs on which resource block(s) theyshould expect the RS.

The invention claimed is:
 1. A user equipment for reporting channelstate information (CSI) to a base station in a communication system inwhich a plurality of downlink component carriers and at least one uplinkcomponent carrier are configured, the user equipment comprising: areceiver, which, in operation, receives from the base station, in a slotn_(Trigger), a trigger message that triggers reporting of the CSI for atleast one unlicensed downlink component carrier of the plurality ofdownlink component carriers; and a transmitter, which, in operation,transmits to the base station, in a slot n_(Report) later thann_(Trigger), the CSI for the at least one unlicensed downlink componentcarrier based on reference signals present, in a slot n_(RS) indicatedby the base station, on the at least one unlicensed downlink componentcarrier, wherein, responsive to the at least one unlicensed downlinkcomponent carrier being occupied for a period of time for a burstydownlink transmission between the base station and the user equipment,only the reference signals in the slot n_(RS) are evaluated for the CSI,and other slots in the period of time for the bursty downlinktransmission are not evaluated for the CSI.
 2. The user equipmentaccording to claim 1, wherein the CSI includes: a wideband channelquality indicator (CQI) value per codeword, which is calculated assuminguse of a single precoding matrix in all sub-bands and assuming downlinktransmission on a set of sub-bands (S); and a selected single precodingmatrix indicator (PMI), or a first PMI and a second PMI corresponding tothe selected single PMI, wherein, the single PMI is selected from acodebook subset assuming downlink transmission on the set of sub-bands(S), and the PMI and the CQI values are calculated conditioned on areported rank indicator (RI) or are calculated conditioned on RI=1. 3.The user equipment according to claim 2, wherein the trigger messageindicates that the reference signals are present in the set of sub-bands(S), which spans a downlink system bandwidth of the at least oneunlicensed downlink component carrier.
 4. The user equipment accordingto claim 2, wherein the transmitter transmits the CSI including one ortwo CQI value(s) for the set of sub-bands (S), which spans a downlinksystem bandwidth of the at least one unlicensed downlink componentcarrier.
 5. The user equipment according to claim 1, wherein thereceiver receives the trigger message in a downlink control information(DCI) format.
 6. The user equipment according to claim 1, wherein thetransmitter transmits the CSI aperiodically, thereby defining anaperiodic CSI report.
 7. The user equipment according to claim 1,wherein the receiver receives the trigger message in the slotn_(Trigger) on another one of the plurality of downlink componentcarriers, which is different from the at least one unlicensed downlinkcomponent carrier.
 8. The user equipment according to claim 1, whereinthe reference signals are at least one of: cell-specific referencesignals (CRS); or channel state information reference signals (CSI-RS).9. A method implemented by a user equipment for reporting channel stateinformation (CSI) to a base station in a communication system in which aplurality of downlink component carriers and at least one uplinkcomponent carrier are configured, the method comprising: receiving fromthe base station, in a slot n_(Trigger), a trigger message that triggersreporting of the CSI for at least one unlicensed downlink componentcarrier of the plurality of downlink component carriers; andtransmitting to the base station, in a slot n_(Report) later thann_(Trigger), the CSI for the at least one unlicensed downlink componentcarrier based on reference signals present, in a slot n_(RS) indicatedby the base station, on the at least one unlicensed downlink componentcarrier, wherein, responsive to the at least one unlicensed downlinkcomponent carrier being occupied for a period of time for a burstydownlink transmission between the base station and the user equipment,only the reference signals in the slot n_(RS) are evaluated for the CSI,and other slots in the period of time for the bursty downlinktransmission are not evaluated for the CSI.
 10. The method according toclaim 9, wherein the CSI includes: a wideband channel quality indicator(CQI) value per codeword, which is calculated assuming use of a singleprecoding matrix in all sub-bands and assuming downlink transmission ona set of sub-bands (S); and a selected single precoding matrix indicator(PMI), or a first PMI and a second PMI corresponding to the selectedsingle PMI, wherein, the single PMI is selected from a codebook subsetassuming downlink transmission on the set of sub-bands (S), and the PMIand the CQI values are calculated conditioned on a reported rankindicator (RI) or are calculated conditioned on RI=1.
 11. The methodaccording to claim 10, wherein the trigger message indicates that thereference signals are present in the set of sub-bands (S), which spans adownlink system bandwidth of the at least one unlicensed downlinkcomponent carrier.
 12. The method according to claim 10, wherein the CSIincludes one or two CQI value(s) for the set of sub-bands (S), whichspans a downlink system bandwidth of the at least one unlicenseddownlink component carrier.
 13. The method according to claim 9, whereinthe trigger message is received in a downlink control information (DCI)format.
 14. The method according to claim 9, wherein the CSI istransmitted aperiodically, thereby defining an aperiodic CSI report. 15.The method according to claim 9, wherein the trigger message is receivedin the slot n_(Trigger) on another one of the plurality of downlinkcomponent carriers, which is different from the at least one unlicenseddownlink component carrier.
 16. The method according to claim 9, whereinthe reference signals are at least one of: cell-specific referencesignals (CRS); or channel state information reference signals (CSI-RS).